The present invention relates to surgical needles, and in particular to needles having a desirably high combination of stiffness, bend strength and ductility.
Surgical needles have been known for some time and such needles with sutures attached are commonly used in various types of surgical procedures. The surgical needles are usually made from stainless steel and have a blunt end to which a suture is attached. The blunt end may be drilled or have a channel placed therein and the suture is usually swaged or crimped in the drilled hole or channel. In order to reduce trauma in placing sutures in tissue, the suture itself generally has a diameter less than the diameter of the needle. Also, the point and cutting edge of the needle should be made as sharp as possible so as to require as little force as possible to penetrate the tissue and place the suture and consequently to cause the least amount of trauma to the sutured area.
Surgical needles are straight or curved, that is, they have the shape of some part of a circle. This may be from a quarter of a circle to five-eighths of a circle. At one end of the needle is the point or the extreme tip of the needle. The section from that point to the maximum width of the needle is termed the "blade" of the needle. In cutting needles, the blade includes the cutting edge from the point towards the blunt end. Behind the cutting edge is the body portion of the needle; that is, the area of the needle to be grasped by an appropriate needle holder. At the blunt end of the needle is the swage or the portion of the needle into which the suture is inserted and affixed. In placing the needle, the needle is grasped by a suitable needle holder which usually is a forceps type instrument that grasps the needle at the body portion with sufficient force to insure that the needle will not move or turn in the instrument when the needle is being placed by the surgeon.
Use of a needle can exert stressing forces on the needle, since the force used to drive the needle into and through tissue (e.g., a blood vessel, the cornea of the eye, and the like) needs to be sufficient to drive the needle and overcome frictional drag through the tissue. Applying this force along the shaft of the needle from the point at which the needle is held risks causing the needle to flex, which is undesirable as it causes loss of control of the needle. This means the body portion of the needle should have a relatively high stiffness, that is, a low tendency to flex and high tendency retain its configuration when subjected to a deforming force. The needle should also not be brittle; if the body is too brittle it may break during use if too much force is placed on the needle during its use. The needle should also be ductile.
The cutting edge and the point of the needle should also be as sharp as possible. The harder the needle the sharper it can be made. The sharper the needle, the less force required to make the initial penetration and the less the drag by the needle body during the remainder of the passage of the needle through tissue. Generally speaking, to make a sharp needle one needs a very hard metal; however, the harder the metal the more brittle it becomes and the greater the chance it will be broken by the needle holding instrument or during placement.
The desirable bend properties for a surgical needle are high stiffness, strength, and ductility in order to penetrate tissue which is being sutured without undue flexing, bending, or breaking during the surgical procedure. Flexing is described as the elastic (or temporary) deflection of the needle out of its original curvature. Stiffness is characterized by the bend modulus which is the slope of the linear-elastic portion of the load-deflection curve. Bending is described as the occurrence of plastic (or permanent) deformation. Strength is characterized by the surgical yield strength which is the point at which the load deflection curve departs from linearity. The surgical yield strength is determined in accordance with the techniques described in Bendel, L. and Trozzo, L., "Tensile and Bend Relationships of Several Surgical Needle Materials", J. of Applied Biomaterials, Vol. 4, pp 161-167 (1993). Breaking is described as the separation of the needle into two pieces. Ductility is characterized as the ability to bend without breaking.
Surgery, especially microsurgery, requires that the needle's path be closely controlled. If the needle flexes or bends as it enters the tissue (or as it pierces the inner surface of e.g., a blood vessel or cornea before re-emerging) there can result improper placement of the needle and suture, and serious trauma to the tissue and the patient. Microsurgery necessarily involves the use of needles of exceedingly small diameter, yet the need for high strength is all the more compelling even as the difficulty of obtaining high strength and high stiffness increases due to the very small dimensions of the needle.
The design techniques generally employed to achieve the described properties of strength and stiffness are often in conflict, however. One straightforward approach to improve the strength of a needle, for instance, is to increase its diameter, or thickness. But by increasing the thickness of the needle, the force necessary to penetrate the tissue is also increased, and the opening left in the tissue after passage of the needle is also enlarged. Likewise, penetration ease can be improved by making the needle thinner, but this approach can risk correspondingly reducing the needle's strength and stiffness. Thus, the design of a needle with favorable performance often requires that a tradeoff be made between size and properties. Bend properties, such as stiffness and surgical yield strength are related to the needle geometry and the material tensile properties: tensile modulus of elasticity and tensile yield strength. Arriving at a satisfactory combination of high strength, high stiffness, and good ductility though, has proven notably elusive.
In general, surgical needles of the type described have been made of various stainless steels. Exemplary of such steels are AISI Type 420 stainless steel and ASTM 45500 stainless steel. The assignee of the present invention also markets needles under the name ETHALLOY, and has disclosed stainless steel compositions useful in making surgical needles in its U.S. Pat. No. 5,000,912. Attempts to develop satisfactory combinations of properties have centered around adjusting the alloy elements of the stainless steel used therein.
Consideration of other materials from which surgical needles might be fabricated is problematic because of the inability to predict whether a composition would exhibit a favorable combination of properties from knowledge of the properties of its components. For instance, tungsten is considered to have a high tensile modulus of elasticity but a relatively moderate tensile yield strength and ductility. On the other hand, noble metals including rhenium, rhodium and iridium (among others) which have a high tensile modulus of elasticity are considered not to have high tensile yield strengths.
There is thus a need for surgical needles of novel composition which permit the economical fabrication of needles and the realization of the properties described herein in combination to a degree superior to those presently available.